Hybrid seesaw leptogenesis and TeV singlets

Abstract

The appealing feature of inverse seesaw models is that the Standard Model (SM) neutrino mass emerges from the exchange of TeV scale singlets with sizable Yukawa couplings, which can be tested at colliders. However, the tiny Majorana mass splitting between TeV singlets, introduced to accommodate small neutrino masses, is left unexplained. Moreover, we argue that these models suffer from a structural limitation that prevents a successful leptogenesis if one insists on having unsuppressed Yukawa couplings and TeV scale singlets. In this work we propose a hybrid seesaw model, where we replace the mass splitting with a coupling to a high scale seesaw module including a TeV scalar. We show that this structure achieves the goal of filling both the above gaps with couplings of order unity.The necessary structure automatically arises embedding the seesaw mechanism in composite Higgs models, but may also be enforced by new gauge symmetries in a weakly-coupled theory. Our hybrid seesaw models have distinguishing features compared to the standard high scale type-I seesaw and inverse seesaw. Firstly, they have much richer phenomenology. Indeed, they generally predict new TeV scale physics (including scalars) potentially accessible at present and future colliders, whereas weakly-coupled versions may also have astrophysical and cosmological signatures due to the presence of a light Nambu-Goldstone boson coupled to neutrinos. Secondly, our scenario features an interesting interplay between high scale and TeV scale physics in leptogenesis and enlarges the range of allowed high scale singlet masses beyond the usual ∼ 10 9 − 10 15 GeV, without large hierarchies in the Yukawa couplings nor small mass splitting among the singlets.

Natural seesaw and leptogenesis from hybrid of high-scale type I and TeV-scale inverse

“…where we have taken √ g * ∼ 10. This analytic estimation was first obtained in our earlier paper [5]. Clearly, a TeV scale m Ψ will result in a too small asymmetry compared to the observed value Y obs ∆B ≈ 9 × 10 −11 [25].…”

“…In particular, in ref. [5] we emphasized that high scale leptogenesis with anarchic couplings can be realized for M N ∼ 10 11 − 10 16 GeV. In this paper, we will study this scenario in more detail and also explore the lower scale M N 10 6 GeV where leptogenesis can be realized albeit with hierarchical Yukawa couplings (among different generations of N ).…”

“…2 Unfortunately, as we have shown recently in ref. [5] (focusing on the case of strong washout) leptogenesis is not achieved naturally in the ISS (i.e. µ = 0 and y = 0), 3 according to our naturalness criteria.…”

“…Finally, table 1 includes the Hybrid seesaw first presented in ref. [5]. This was designed to overcome simultaneously the two limitations of the ISS: unnaturally small µ term and difficulty in leptogenesis.…”

“…Regarding possible experimental signals in the hybrid seesaw, besides signals associated with TeV scale fermions as in the conventional ISS (as mentioned above), the model generally predicts new TeV scale scalars potentially within the reach of present and future colliders as we have shown in ref. [5]. Certain realizations, like the gauge model presented in appendix E, also contain light states that may contribute to ∆N eff and might thus be probed by CMB-Stage-IV [17].…”